Adds start and stop work heartbeats. (#1188)

* Adds start and stop work heartbeats.

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Fix code after suggested changes

* Finalizing stops work on earlier lower blocks.

* Fix func parameter and flaky test

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

polkadot/overseer/examples/minimal-example.rs

@@ -111,6 +111,7 @@ fn main() {
 		});
 
 		let (overseer, _handler) = Overseer::new(
+			vec![],
 			Box::new(Subsystem2),
 			Box::new(Subsystem1),
 			spawner,